This invention relates to programmable electronic devices in general and particularly to electronic devices such as radios having keypads which can be used for programming purposes.
Radio designs, as for example, channelized two-way radios, have changed dramatically in recent years with advances in frequency synthesizers, memories, microprocessors and digital control technology. In the past, radio frequencies were typically controlled by crystal channel elements, options and features such as scanning, coded squelch, tone encoders, and the like were typically implemented at the time of manufacture and could only be added to a radio or changed by making internal changes or adding modules. The use of frequency synthesizers permits radios to be programmed for operating frequencies, thereby eliminating the necessity of crystals for every operating frequency. The use of digital control circuits and, particularly, microprocessor based control of radios has provided a great deal of flexibility in radios as to the features and modes of operation. Such features as scanning, as well as coded squelch operation can be easily implemented and changed to suit the user in microprocessor based radios. The use of EEPROM's (Electrically Erasable Programmable Read Only Memories) that can be electrically reprogrammed within the radio now provide a means for non-volatile storage of radio program information. Alternatively, RAM'S (Random Access Memories) either using special memory backup batteries or with memory saving circuitry, that allow for a change of radio batteries without loss of memory, can also be used to maintain radio program information. Radios using such memories can be programmed by external programmers connected to the radio via a programming port. However, greater convenience and flexibility is achieved by use of a keypad on the radio for programming purposes. Such use avoids both the expense and necessity of having a specialized electronic programmer available for programming the radio.
There are, however, drawbacks to the use of keypads for programming purposes. This approach may be undesirable from the standpoint of the possibility of a radio user either unintentionally or without authorization changing the radio programming. For instance, where the user is licensed to operate on certain frequencies, it is desirable to provide means for preventing the user from reprogramming the radio to operate on unauthorized frequencies. Where the radio is provided with memories for generating DTMF (dual-tone multifrequency) signals as for accessing a telephone system, it may be desirable to prevent the user from inadvertantly reprogramming the phone number memory. Consequently, while it is desirable to be able to utilize the radio keypad for programming, a method of preventing the user from intentionally or unintentionally altering the programmed information is highly desirable.